In recent years, LED power diodes have emerged on the market which may find possible use in the lighting sector, for example in street lights. Such power diodes are lighting sources with supply currents up to 1.5 A and nominal operational voltages around 3.5. With respect to the power supply for such applications, the LED diodes require a controlled and constant current, which influences the brightness and color temperature.
A particularly notable feature of these LEDs is their long life of approximately 50,000-100,000 hours, which allows for a long lasting and low maintenance lighting apparatus.
There is a need for power supply systems for such LED diodes which will have an operating life similar to that of the LED diodes. However, this object is not simple to achieve. For example, in the particular case of public/street lighting, the power supply systems have to be supplied by the public energy grid, are used outdoors, and therefore are subject to stressful conditions such as temperature, voltage peaks, lightning strikes, mechanical stresses, etc.
Still with reference to the particular sector of public/street lighting, in some public lighting systems, remote control of the activation and deactivation of each lighting apparatus, brightness adjustment for energy saving reasons, and collecting of operating data for diagnostic or other reasons is required. A technique which is increasingly popular for achieving these goals is the use of a power line modem (PLM), i.e., a modem for receiving and transmitting data which uses as a physical carrier the same power supply cables of the lighting apparatus. There are clear economic advantages to such a technique.
With reference to FIG. 1, showing the functional block diagram of a lighting apparatus 1 with LED diodes 2, the above said technique is problematic in that the transceiver port of PLM modem 3 and input of supply and switching system 4 of diodes 2 are coincident. Indeed, they are parallel-connected in order to be supplied with the grid voltage VAC.
Moreover, the power supply system 4 when operating in switching mode, emits electrical noise at its switching frequency and harmonics, as well as at oscillation frequency of parasite resonant circuits in the system, stimulated by switching and respective harmonics. Furthermore, the simultaneous presence of harmonics at different frequencies gives rise to intermodulation effects, which generates further harmonic components. The harmonics of this electrical noise which are within the frequency band used by PLM modem 3 may damage the transmitted signal, or worse, the received signal, which has a lower amplitude, since it is attenuated by the propagation path.
In order to avoid this, it is necessary to provide filters 5, 6 which minimize the noise which the supply system 4 backwardly injects through the input terminals, as well as the noise entering from the input port of PLM modem 3. It is also necessary to minimize the direct coupling between the switching supply system 4 and PLM modem 3.
FIG. 2 shows the functional block diagram of a power supply system 4 of the known art for supplying power LED diodes 2. The power supply system 4 comprises a PFC (Power Factor Corrector) pre-regulator 5 so as to receive from the power grid a substantially sinusoidal current which is in phase with grid voltage VAC. Typically, the PFC pre-regulator 5 is comprised of a boost converter for generating an approximately continuous voltage over the output capacitor 8, starting with a rectified grid voltage VAC.
The power supply system 4 also comprises a DC-DC converter 6 for converting the output voltage VIN of PFC pre-regulator 5 into a continuous voltage VOUT of suitable value for supplying the LED diode group, at the same time providing the insulation required by safety regulations. Also for safety reasons, it is often required that the output voltage VOUT is not higher than 60 V even in case of failure, in order to comply with SELV (Safety Extra Low Voltage) specifications. A typical value is 48 V. In the switching power supply system 4 there is also provided a feedback control loop 8, 9 associated with the DC-DC converter 6 in order to keep the output voltage VOUT at the specified value, with respect to a variable current absorbed by the load and input voltage VIN. Various topologies for DC-DC converter 6 may be used; however, in order to obtain the maximum conversion efficiency possible, resonant DC-DC converters 6 are employed.
The switching power supply system 4 also comprises one or more current regulators 7, each associated with a respective string of series-connected LED diodes, the strings being parallel-connected to each other. The reason for using various strings of parallel-connected LED diodes is that with voltages of 48 V and using common 1 A LEDs, it is not possible to provide power above approximately 50 W, whereas in the case of street lighting, higher power levels are often required (up to 150-200 W). In order to achieve such power levels, it is therefore necessary to provide many parallel-connected strings of series-connected LED diodes. However, the voltage drop on each string of LED diodes is highly variable with respect to production tolerances. Therefore, if these would be directly parallel-connected to the output of DC-DC converter 6, the string showing the least drop would absorb all the power provided by the DC-DC converter 6, whereas the other would remain switched off. This is obviously unacceptable; the current of LED diodes has to be the same for all the LED diodes strings in order to provide an uniform lighting. To this end, a current regulator 7 is provided for each string. This not only regulates the current at the desired value, but also acts as a “damper”, absorbing the difference between voltage provided by DC-DC converter 6 and the one present at the ends of each string.
With reference to FIG. 3, such current regulators are, for example, comprised of a non insulated switching converter and a feedback current control loop. The current regulator 7 of FIG. 3 is, for example, provided with a monolithic regulator 10 (i.e., the control section and power section all on the same board) at a constant frequency and with PWM modulation (pulse width modulation). In the example of FIG. 3, the monolithic regulator 10 is an integrated circuit produced by STMicroelectronics and is sold as model L6902D. In FIG. 3, reference 11 indicates a string of five LED diodes 2, whereas the terminal 12 is the output terminal of DC-DC converter 6 of FIG. 2.
A power supply system 4 of the state of the art of above said type for use within a lighting apparatus or fixture, when provided with a communication system with PLM modem 3, has some drawbacks which are described below.
The boost PFC pre-regulator 5 for managing the expected power levels uses control techniques which render its switching frequency variable in time with instantaneous grid voltage in a value range which depends from both the grid effective voltage and load. It may be almost impossible to ensure that neither the fundamental component nor any of its harmonics fall within the transmission band of PLM modem 5.
In the specific case wherein the DC-DC converter 6 is a resonant converter, the regulation of output voltage VOUT is obtained by varying the frequency according to the input voltage and load. In the supply system 4, since the input voltage of DC-DC switching converter 6 is the voltage output by PFC pre-regulator 5, the operating frequency changes according to input voltage are limited but non-zero, since in the output voltage of pre-regulator 5 an alternating component is present, with a frequency which is two times the grid voltage VAC frequency superimposed to a continuous voltage value. This causes a modulation of operating frequency of DC-DC switching converter 6. The frequency variations due to load variations remain the same (e.g., in case of lighting adjustments). These operating frequencies are different from those of the pre-regulator 5, so that also the intermodulation harmonics are present, which may be within the transmission band of PLM modem 3.
The output voltage of the DC-DC switching converter 6 is regulated by means of a feedback control loop 8, 9 which compares the output voltage with a reference value, and through the generated error signal, changes the operating frequency of DC-DC switching converter 6. In doing this, the error signal has to cross the insulating barrier between the input and output of DC-DC switching converter 6, required by safety regulations. In order to let the error signal cross the insulating barrier, a photo-coupler 9 is typically employed. This has a limited average life span, unless a very expensive device is used, and this negatively influences the life span of the whole supply system 4. Moreover, in case of lightning strike, the photo-coupler 9 is one of the components most sensible to failure.
Another important element of a supply system 4 of above type is the electrolytic capacitor 8 at output of PFC pre-regulator 5. The electrolytic capacitor 8 is subject to a considerable current stress of a low frequency component (two times the grid frequency) due to the fact that, measured in the scale of the grid frequency, a PFC pre-regulator 5 behaves like a floating power generator at high frequency, due to the combined effect of switching by PFC pre-regulator 5 and of downstream DC-DC switching converter 6.
Accordingly, there remains a need for a switching power supply system which obviates above drawbacks of known power supply systems.